1. Field of the Invention
The present invention relates to a process for producing a monoalkenyl aromatic hydrocarbon compound. More particularly, it pertains to a process for producing a monoalkenyl aromatic hydrocarbon compound by alkenylating a side chain of an aromatic hydrocarbon compound having at least one hydrogen atom bonded to an .alpha.-position of the side chain by using a conjugated diene having 4 or 5 carbon atoms in the presence of an alkali metal-based catalyst, and separating the objective monoalkenyl aromatic hydrocarbon compound in the reaction product to recover the same with high purity in a high recovery rate.
2. Description of Related Arts
As a catalyst for producing a monoalkenyl aromatic hydrocarbon compound by alkenylating a side chain of an aromatic hydrocarbon compound by using a conjugated diene having 4 or 5 carbon atoms, there is known a catalyst comprising an alkali metal such as sodium and potassium or an alloy thereof.
It is known that in the case where the objective monoalkenyl aromatic hydrocarbon compound is recovered by separation from the reaction product in liquid form obtained by the use of such a catalyst, an attempt to separate and recover the objective monoalkenyl aromatic hydrocarbon compound after the completion of the reaction by allowing the reaction product to cool, separating the liquid phase containing the objective product from the catalyst through decantation or filtration, and distilling the separated liquid phase results in failure to obtain the objective monoalkenyl aromatic hydrocarbon compound with high purity in high yield, since the objective compound changes in quality and properties during such steps. In order to solve such a problem there are proposed various methods for treating the reaction mixture.
There is proposed in Japanese Patent Application Laid-Open No. 4127/1976, a method in which the concentration of the total amounts of alkali metals and organic alkali metal compounds in the hydrocarbon compound to be used as the starting raw material for distillation is set to the range of 0.09 to 15 mg-atom expressed in terms of alkali metal components per one (1) kg of the hydrocarbon compound. In the above-mentioned method, however, it does not follow that all the alkali metal catalyst components are removed, but part of the components are inevitably mixed in the liquid phase in the form of a soluble alkyl compound or an alkenyl complex. In the case of the reaction product being introduced as it is into a distillation column in the method, it occurs that the monoalkenyl aromatic hydrocarbon compound further reacts with itself or with an unreacted alkylaromatic hydrocarbon compound to form a high molecular byproduct owing to the coexistence of the active catalyst. The objective product is converted to the alkylaromatic hydrocarbon compound as the starting raw material by the reverse reaction, or an isomer other than the objective monoalkenyl aromatic hydrocarbon compound is formed by the transfer of the double bond. Even if the concentration of the total amounts of alkali metals and organic alkali metal compounds are lowered, the change in quality and properties of the objective compound can be lessened to some extent, but can not be entirely prevented. When the distillation column is operated for a long period of time, the alkali metals and organic alkali metal compounds each in a slight amount are concentrated in the distillation column, finally making it impossible to obtain the objective monoalkenyl aromatic hydrocarbon compound with high purity and lowering the recovery rate.
The change in quality and properties of the objective compound can be suppressed to some extent by lowering the distillation temperature, which however, makes it necessary to carry out the distillation under reduced pressure. In order to completely eliminate the change in quality and property of the objective monoalkenyl aromatic hydrocarbon compound, the above-mentioned method requires a high vacuum and accordingly, it is far from an industrially economical process.
There is proposed in Japanese Patent Application Laid-Open No. 70929/1974, a method in which the catalyst is removed by separating it from the reaction product. Subsequently the resultant reaction liquid is treated with carbon dioxide, followed by distillation or. Alternatively, the reaction product is treated with carbon dioxide and then, the catalyst is removed by separating it from the reaction product, followed by distillation. The aforesaid method can prevent the change in quality and properties of the objective monoalkenyl aromatic hydrocarbon compound in the course of distillation since the alkali metal and organic alkali metal compound are completely inactivated by the carbon dioxide treatment. However, this method brings about the disadvantage that some of the formed alkali salts such as alkali carbonates and alkali carboxylates that are soluble in organic solvents are introduced into a distillation column, where the unreacted aromatic hydrocarbon is distilled away, and the alkali salts beyond this solubilities are precipitated and accumulated in solid form inside the distillation column, thereby lowering the gas-liquid contact efficiency and distillation efficiency in the distillation column and finally making it impossible to proceed with the distillation procedure because of clogging in the column. That is to say, unless the alkali metal components are completely removed from the system, it is impossible to operate the distillation column in a stable manner and obtain the objective monoalkenyl aromatic hydrocarbon compound with high purity.
There is proposed in Japanese Patent Publication No. 26489/1982, a method in which an alkenyl aromatic hydrocarbon compound is brought into contact with water and the pH of the water phase is adjusted to 6 or less to make the alkali metal catalyst water-soluble, followed by separation of the catalyst. However, the above-mentioned method involves the danger of causing fire in addition to the generation of a large amount of reaction heat in the case of bringing the reaction product into contact with water. Although it is possible to control the reaction to some extent by regulating the amount of water or reaction product in liquid form to be brought into contact, such treatment requires a long period and a large-scale equipment when carried out on industrial scale and proves unpractical.
In U.S. Pat. No. 3,244,758, the occurrence of an unfavorable side reaction in the course of distillation is prevented by inactivating the alkali metals and alkali metal compounds contained in the reaction product in liquid form through the addition of isopropanol prior to distillation. Although the aforesaid method can avoid the danger of causing fire in addition to the generation of a large amount of reaction heat in the case of contact between the reaction product with water as mentioned above, but the inactivated alkali metal catalyst in the form of an alkali metal alcoholate which is soluble in organic solvents is introduced into a distillation column, thus unfavorably causing the problem as is the case with the foregoing.
As described hereinbefore, there has not yet been discovered any suitable method for removing alkali metal components in the reaction product when an alkali metal or an alloy thereof is employed as a catalyst in the alkenylation reaction.
On the other hand, it has been found that an alkali metal supported on a carrier other than the above-described alkali metals or alloys thereof is effective as a catalyst for alkenylation reaction. It has also been found that among various alkali metal catalysts each supported on a carrier, the catalysts prepared by the treatment as described hereunder exhibits not only extremely high activity in alkenylation reaction but also remarkably low inflammability. Specifically, the above-mentioned high activity and low inflammability are assured by the use of the carrier obtained by calcining a mixture of potassium hydroxide and aluminum hydroxide; the carrier obtained by calcining the mixture of a basic potassium compound and alumina; the alkaline earth metaloxide carrier containing a potassium compound; the zirconium oxide carrier containing a potassium compound; or the alkali metal-based catalyst supported on a carrier obtained by heat treating any of the aforesaid carriers along with metallic sodium in an atmosphere of an inert gas. However, there has not been, in fact, proposed a treatment method in the case of employing the above-mentioned alkali metal-based catalyst supported on a carrier.
Under such a circumstance, an attempt was made by the present inventors to recover a monoalkenyl aromatic hydrocarbon compound by method the following. The reaction product obtained by the use of the aforestated alkali metal-based catalyst supported on a carrier is allowed to stand and be filtered to separate the liquid phase containing the objective product from the catalyst, and the resultant reaction liquid is distilled to separate the objective product. As a result, it was impossible to obtain the objective monoalkenyl aromatic hydrocarbon compound with high purity in high recovery rate, since the objective compound changed in quality and properties during the course of distillation. In addition, insoluble compounds were accumulated in the distillation column. As a result, the column was clogged by the accumulated mater and thus it was impossible to continue with the distilling (distillation) operation the column.
That is to say, it was impossible to completely remove the catalyst in the form of fine particles by allowing the reaction product to stand, followed by filtration, and part of the alkali metal is mixed in the liquid phase as a soluble alkyl or alkenyl complex. When the reaction product containing such a complex is introduced as it is in a distillation column, the coexisting active catalyst causes the monoalkenyl aromatic hydrocarbon compound to further react with itself or with an unreacted alkyl hydrocarbon compound resulting in the formation of a high molecular byproduct, which is converted by reverse reaction into the alkyl aromatic hydrocarbon compound, i.e., the starting raw material, or to form an isomer other than the objective monoalkenyl aromatic hydrocarbon compound by the transfer of the double bond. Consequently, it is impossible to produce the objective monoalkenyl aromatic hydrocarbon compound with high purity, and the recovery rate is lowered.
The change in quality and properties of the objective monoalkenyl aromatic hydrocarbon compound can be suppressed to some extent by lowering the distillation temperature as mentioned hereinbefore, which however, necessitates to performing the distillation under reduced pressure. In order to completely eliminate the change in quality and properties of the objective compound, the above-mentioned low temperature method requires a high vacuum and thus, it is far from industrially practical because of the expensiveness in equipment and operation.
Moreover, some alkali metals in the form of alkali salts that are soluble in organic solvents are introduced in a distillation column, where the unreacted aromatic hydrocarbon is distilled away, and the alkali salts beyond their solubilities are precipitated and accumulated in solid form inside the distillation column, thereby lowering the gas-liquid efficiency and distillation efficiency in the column and finally making it impossible to proceed with the distillation procedure because of clogging in the column. Thus the above-mentioned problem can not be avoided by running conditions only at the time of distillation.